Colors displayed by a display apparatus comprise color components of red, green, and blue. This is called an RGB format, the title of which is obtained by collecting initial letters of the color components. On the contrary, colors printed by a printer comprise mainly color components of cyan, magenta, yellow, and black. This is called a CMYK format, the title of which is obtained by collecting initial letters of the color components. In the RGB format of the display apparatus, the brightness of the respective colors comprises 256 gray scales and the concentration of the respective colors is expressed by the gray scales. On the contrary, in the CMYK format of the printer, the brightness of the respective colors comprises 2 gray scales of existence and non-existence of a toner of the corresponding color component and the concentration of the respective colors is expressed by the amount of toner. That is, the colors of the display apparatus are expressed in a multi-valued RGB format and the output of the printer is expressed in a two-valued CMYK format.
When the colors displayed by the display apparatus are printed by the printer, the colors expressed in the multi-valued RGB format should be converted into the two-valued CMYK format. In this case, generally, the multi-valued RGB format is not converted directly into the two-valued CMYK format, but through a procedure of multi-valued RGB→multi-valued CMYK→two-valued CMYK.
There is known a method of converting the multi-valued CMYK data into the two-valued CMYK data by the use of a dither pattern. In this method, the dither pattern is prepared in advance, the multi-valued data to be converted are compared with the dither pattern, a value of the multi-valued data is converted into 1 when the value is greater than or equal to a threshold value written in the dither pattern, and a value of the multi-valued data is converted into 0 when the value is less than the threshold value. FIG. 12A illustrates one example of a 4×4 dither pattern. By performing the conversion by the use of the dither pattern shown in FIG. 12A, the multi-valued data in the left side of FIG. 12B are converted into the two-valued data in the right side of FIG. 12B.
Practically, when the multi-valued data of pixels of a row of image data are converted into two-valued data, the threshold values of a row of the dither pattern are used. That is, the values of the pixels of a row of the image data are read out for every pixel, the threshold values of a row of the dither pattern are sequentially read out from the front end (the left end in FIG. 12A), and then the read-out values are compared with each other, thereby performing the two-valued conversion process. When the threshold value of the tail in the corresponding line of the dither pattern is read out, the threshold value of the head in the same line is read out again. That is, a readout position of a threshold value of a row of the dither pattern periodically varies. Accordingly, a periodically varying value is generated by a counter, and the readout position of a row of the dither pattern is generated on the basis of the value of the counter, as disclosed in JP-A-7-262349.